In a typical dual-beam headlamp there are two filaments that are located in carefully-selected, spaced locations within the usual reflector of the headlamp. One of the filaments is energized to produce a lower beam, and the other is energized to produce an upper beam. The spatial separation between the two filaments is relied upon to create the desired beam shift between the upper and lower beams. The lower beam is normally aimed as shown in FIG. 1 of the drawings of the present application, and the upper beam is aimed as shown in FIG. 2 of these drawings. These FIGS. 1 and 2 depict a screen located in a target plane in front of a vehicle, the screen being viewed from the vehicle with the headlamp aimed at the screen.
When the lower beam is aimed as in FIG. 1, the top portion of its high intensity zone is at the headlamp's "horizontal" (i.e., a horizontal line 8 located at the point where the headlamp's center line intersects the target plane.) The leftmost edge of the high intensity zone of the beam of FIG. 1 is at the headlamp's "vertical" (i.e., a vertical line 9 located at the point where the headlamp's center line intersects the target plane).
The upper beam, on the other hand, is aimed so that the central area of its high intensity zone is aimed at the intersection of the headlamp's horizontal and vertical, as seen in FIG. 2. Thus, by design, the upper beam is shifted up and to the left of the lower beam, as viewed from the vehicle. As viewed looking into the headlamp, such shifting is up and to the right.
Given that the reflector is of a parabolic shape and has a predetermined focal length, one can determine that a given source displacement (upper-beam source position versus lower-beam source position) will yield a predictable beam displacement. Accordingly, the filaments in a dual filament headlamp have been designed with a controlled lateral displacement (offset) from the headlamp center line and a controlled vertical displacement (spacing) from this center line.
A headlamp that utilizes a discharge-type light source has design characteristics unlike the incandescent headlamp referred to above. Instead of relying upon filaments, as above described, the discharge-type light source relies upon an arc, or arcs, within an arc chamber to generate the desired light. The operation of this type of lamp is materially affected by the shape and size of the arc chamber. One might approach this problem by providing a quartz arc chamber and means for developing within the chamber two arc paths analogous to the two filaments used to create the above-described lower beam and upper beam. This approach would require within the arc chamber dual sets of electrodes, along With associated lead wires and control circuits. Such a lamp would be difficult to design and manufacture in view of the large number of parts that would have to be sealed to the quartz arc chamber and the necessity for a high degree of precision in locating such parts relative to each other and in shaping the arc chamber.
An alternate approach employs a dual reflector system, each reflector having its own arc source and starting circuit. A significant problem with such a scheme is that the additional reflector and arc source call for an additional arc tube and mount hardware and additional electronics and circuitry, and this has a significant adverse effect on product size and cost.
The use of two arc tubes in a single reflector would overcome some of the cost penalties of duplicate reflectors. However, the physical size of the arc tubes prevents placing them in close enough proximity, typically about 0.05 inches to 0.15 inches, required to maintain the proper relationship between the upper and lower beams.